This invention relates to a system and method for in-line heating of medical fluid supplied to a patient.
In numerous medical procedures it is necessary to supply medical fluid to a patient. The medical fluids often are blood or blood products. Saline, anesthetics and other medical fluids are also commonly supplied to patients undergoing medical procedures.
Warming fluids is a major problem in the operating room particularly in abdominal surgery, prolonged surgery and all major trauma surgery. Because the surgeon is in a gown he prefers the room quite cool (typically 60 degrees F.) to prevent sweat from falling into the wound. The patient, under anesthesia, loses most abilities to regulate his own temperature. Then cold intravenous fluids are infused and the patient becomes colder and colder as the operation proceeds. If major blood loss or "third spacing" of fluids occur, the patient will typically lose several degrees of temperature within two hours. If the patient becomes colder than 34 degrees centigrade, many anesthesiologists will not extubate the patient and prefer to leave the patient intubated and mechanically ventilated because of the deleterious effects of cold on mental status and respiratory muscle strength in the face of resolving neuromuscular blockade. Effects of cold on clotting become more noted at lower temperatures, as does the tendency toward ventricular arrhythmias. Below 32 degrees, the patient stands a great chance of a fatal ventricular arrythmia and certainly problems from inadequate cardiovascular performance. Patients die from this type of cold.
Warming fluids from their storage temperatures (usually 4.degree. C. for blood and blood products) to normothermic or even slightly hyperthermic temperatures is a well-recognized way to deal with this problem. Current recommended practice is to use blood warmers to bring blood to about 37.degree. C., which is body temperature. Unfortunately, for the patients who need it most, the usual fluid warming device fails, because it cannot handle adequately fluid flows greater than 100 cc/min. Typically the conduction heating devices result in fluids of lower and lower temperatures as the flow rate increases.
Common complaints about existing blood warmers is that they have high flow resistance, do not warm the blood enough, are difficult to prepare, are too expensive, too bulky, and present risks of infection of the blood and risks of spillage of blood products in the operating room because of large surface area, thin-walled reservoirs for heat transfer. The poor performance of available equipment contributes to undesired hypothermia of patients, a known cause of delays in awakening after surgery which needlessly increases the (expensive) time the patient must spend being intensively monitored in a recovery room and delays the ability for a full mental and neurologic assessment of the patient to be made after surgery. Cold patients are a real problem from all perspectives.
The existing blood/fluid warmers fall into broad categories of in-line warmers which heat fluid flowing in a path to the patient and batch warmers which heat fluid in a batch, the fluid then being placed in a separate delivery system for supplying the patient. Warmers fall into the following more specific categories:
(a) Water Bath PA1 (b) Dry Heat PA1 (c) Microwave PA1 (d) Radiowave
The first two of the four specific categories are usually inline blood warmers and depend on conduction as the main mode of heat transfer. The latter two categories are usually batch type warmers, i.e., they are not in-line warmers (but an in-line microwave warmer is discussed below). The microwave warmer uses electromagnetic radiation of 2450 MHz to excite the vibration mode of the water molecules to produce heat. The radiowave warmer uses a different frequency (27 MHz), and therefore a longer wavelength of electromagnetic radiation to warm the blood. It is also worth noting that, as far as known, none of the existing commercial blood warmers exploit the powerful flexibility and versatility of microprocessors to control the infusion temperature. The four categories will be discussed in turn.
(a) Water Bath Blood/Fluid Warmers
Blood flows in a coil, made of polythene tubing, that is immersed in a stirred warm water bath at 35-37 degrees C. The water bath is heated by an electric heater element and sensors monitor the temperature of the water bath. In older simpler models, warm water from the tap is used for the water bath and there is no in-built heater. It is surprising to note that the temperature of the blood/fluid at the outlet of the warmer is not monitored. Current blood/fluid warmers of this type are the Level 1 (Patent pending), Dupaco Hemokinethitherm and Jensen 709-100-1. The Level 1 warmer (a counter-current heat exchanger using water at 40.degree. C. as the heating medium) is currently considered as the best in-line blood warmer.
The Level 1 Fluid Warmer is a known commercial product which was developed to meet this need, using an improved conduction heating system. Quite formidable in size, weight and cost, this device also involves the use of a very expensive disposable, limiting its usage for all but the most desperate cases. The disposable is a portion of the system which is in contact with the blood or other fluid and which must be replaced to use the system on a new patient. It claims the ability to warm "cold blood" at 500 cc/min and room temperature solutions at 1000 cc/min. A smaller, somewhat less expensive device and disposable are able to handle half this capacity, or 250 cc/min cold blood. Unfortunately, the disposable still has a very significant price because of its complex heat transfer device.
The warm water bath is a perfect medium at the right temperature for growing bacteria. In a study, cultured samples from water baths yielded Bacillus species in 72% of the samples, Flavobacterium species in 39% and Pseudomonas species in 9%. These bacteria can contaminate the administered fluid or blood by gaining entry at the connections between the tubing and the coil. A fatal case of Pseudomonas septicaemia in a patient given fresh frozen plasma warmed in a water bath contaminated with Pseudomonas aeruginosa is reported in the literature.
The design of a water bath blood warmer uses a circuitous pathway for heating blood: the electric heater element heats the water, the water heats the polythene coil, the polythene heats blood close to the tubing wall and the heat from the blood layers close to the polythene walls is then transferred to the blood in the center of the tube. The multiple layers of thermal resistance interposed between the heat source and the fluid to be warmed decrease the efficiency of the system. Further, polythene is a relatively poor conductor so that there is a significant temperature gradient across the polythene wall.
In the water bath warmer, to ensure that the fluid in the center of the tube is rapidly warmed by the fluid in contact with the tubing wall, the distance between these two layers of fluid is minimized by using small bore tubing (3/16" internal diameter). Since the residence time of the fluid in the water bath must be sufficient for the fluid to absorb enough heat, a long length of tubing is used to generate the required dead space and hence the desired residence time. This long length of small bore tubing causes appreciable flow resistance which limits the flow rate that can be passed through the warmer when using gravity feed. Using a 58 per cent glycerol water solution to simulate blood, the Portex coil (a commonly used coil in water bath warmers) showed a pressure drop of 19 torrs at a flow rate of 22.4 mL/min and 48.3 torrs at 56.5 mL/min. Flow rates of up to 250 mL/min are sometimes required and the pressure drop across the tubing would then be excessively high since flow resistance as well as pressure drop increases with flow rate in real systems.
With the water bath heater, at steady state, the mean infusion temperature of the fluid decreases as the flow rate increases. With cold water at 5.degree. C. at the inlet, the mean outlet temperature dropped from 36.degree. C. at a flow rate of 150 mL/min to 28.degree. C. at 340 mL/min. This steep degradation in performance at increased flow rates can be explained by the shorter residence time of the blood in the water bath at higher flow rates. Less heat can be transferred to a given volume of fluid because there is less time during which heat transfer can take place. It is interesting to note that the plot for mean outlet temperature versus flow rate is linear within experimental accuracy. This is not surprising since mean outlet temperature is proportional to heat transfer which is proportional to residence time which is inversely proportional to flow rate. However, it is alarming that at high infusion flow rates, the degradation in performance of water bath blood warmers means that large volumes of fluid significantly below the recommended minimum infusion temperature (32.degree. C.) are being administered, thus increasing the risk of fibrillation.
The cooling of the fluid as it flows from the warmer to the catheter is not taken into account in the water bath systems.
In a water bath blood warmer, the thermal inertia of a large mass of water is required to damp out temperature fluctuations. Therefore, the system is necessarily bulky, heavy and unwieldy to use. The water bath can spill and overflow, creating slippery and electrically unsafe conditions in the operating room.
The long pathway that the heat takes before getting to the blood creates a pure time delay between the control action (cause) and the response (effect). Pure time delays are undesirable in closed loop control systems since they destabilize the system especially if the time delay is large enough that the error signal is 180.degree. out of phase with the reference input.
(b) Dry Heat Blood/Fluid Warmers
As the name implies, the dry heat blood warmers do not use a water bath. Instead, metal (usually aluminum) surfaces in intimate contact with a plastic cuff (e.g. American Pharmaseal DW-1000) or tube (DataChem Inc. FloTem II, U.S. Pat. No. 4,532,414) are heated by electric heater elements and transfer heat to the fluid by conduction via the plastic. Thermostats monitor the temperature of the metal surfaces in contact with the plastic containing the blood and turn the heaters on and off accordingly. Here again, the outlet temperature of the fluid is not monitored. In general, the performance of dry heat warmers is inferior to the water bath warmers. Other current dry heat blood warmers are the Electromedics BT-794, Fenwal BW-5 and the Mallinckrodt Animec.
Flow resistance is typically high in dry warmers. Since conduction is the main mode of heat transfer in dry heat blood warmers, the cuff or tubing has to be of narrow bore which increases flow resistance and limits the amount of flow available with gravity feed. The cuff used with the Gorman-Rupp DW 1220 blood warmer exhibited 1.8 times the flow resistance of the Portex coil used in water bath blood warmers: 34 torrs pressure drop at an infusion flow rate of 22.4 mL/min and 85.7 torrs at 56.5 mL/min. With the American Medical Systems DW-1000 cuff, the maximum flow rate with gravity feed is 200 mL/min according to the manufacturer. We could get a maximum flow rate of only 150 mL/min with gravity feed from a standard saline bag mounted four feet above the cuff and connected to the blood warmer via a Y-type blood-solution recipient set with large filter (Fenwal code 4C2132).
Heat transfer to the blood is ineffective in a dry warmer. The cuff in the Gorman-Rupp DW 1220 and the tube in the DataChem FloTem II are heated from one side only. In tests conducted with 5.degree. C. water, the heating efficiency of the DW 1220 was found to be inferior to the Portex coil in a warm water bath maintained at 36.8.degree. C. with a thermostat. The mean outlet temperature dropped to 32.degree. C. at a flow rate of 157 mL/min for the DW 1220 and 228 mL/min for the Portex coil. For the FloTem II, the manufacturer's specifications state that when supplied with 4.degree.-6.degree. C. water, the outlet temperature will be 33.degree. C. at a flow rate of 5 mL/min, 29.degree. C. (below the recommended minimum of 32.degree. C.) at 25 mL/min and room temperature of 100 mL/min.
In dry heat warmers, the steady state outlet temperature of the fluid is a function of the flow rate. When supplied with 5.degree. C. water at the inlet, the mean outlet temperature drops from 33.degree. C. at a flow rate of 100 mL/min to 27.degree. C. at 290 mL/min.
Plastic is a poor conductor and causes a large temperature drop between the heated metal surface and the fluid in the cuff or tube in dry warmers. Since the temperature of the heated surface, instead of the outlet temperature of the fluid is being monitored, there is a temperature offset which results in the temperature of the fluid at the outlet being colder than the heated surface.
The cooling of the fluid as it flows from the warmer to the catheter is not taken into account in dry warmers.
The thermal inertia of the heated metal surface and the plastic create pure time delays which degrade the response time of the system to a change in operating conditions. The pure time delays make it difficult to use a closed loop control system with dry heat warmers.
In the dry heat blood warmer, the cuff is made of thin plastic to improve heat transfer. However, this also makes it very easy to rupture the cuff and spill its contents.
(c) Microwave Blood Warmer
The literature on blood warmers mentions two microwave blood warmers (Haemotherm Universal and Haemotherm B), both manufactured by Robert Bosch Elektronik GmbH, Berlin, West Germany. The Haemotherm Universal and Haemotherm B are of similar construction; the only technical difference is that the Universal can warm both blood bottles and bags whereas the B warms blood bags only. The warmers operate at a frequency of 2450 MHz, with a rated power output of 400 W developed by two magnetrons. The blood unit is placed in an insulated chamber where it is continuously mixed by a 350.degree. rotation around its cross-axis to prevent hot spots due to non-uniform distribution of the microwave radiation. A temperature probe on the surface of the bag monitors the blood temperature by inference and switches off the magnetrons when the temperature reaches a preset value. Warming is also interrupted if the stirring mechanism fails or if the chamber is opened.
A study showed that "microwaves per se are not harmful to erythrocytes but that poor penetrance of microwaves, together with insufficient blood mixing during warming, are the critical factors leading to hemolysis."
U.S Pat. No. 3,963,892 of Camph et al issued Jun. 15, 1976 shows in-line heating by microwaves of blood being passed from a container to a patient.
The Haemotherm is a batch heater. The blood unit will cool down to room temperature if it is not immediately administered to the patient or if it is administered at low flow rates.
Blood units are about three inches thick at the widest point and the depth to which microwaves can penetrate in blood is 1/2 to 1 inch when batch microwave heating is performed. To prevent hot spots and ensure that blood in the center of the unit is exposed to microwave radiation, the blood unit needs to be continuously mixed by a rotating action. Even with rotation of the bag, there are hot spots and consequently hemolysis at the corners of the bag because there is flow stagnation at the corners.
The in-line microwave heating of the Camph patent avoids some of the batch heating problems. However, it would be relatively expensive to produce as it apparently would use thermocouple measurement of blood temperature flowing past the microwave. The thermocouple would have to be manufactured as part of the sterile fluid path of the blood to have reasonable degree of accuracy and would have to be calibrated and connected to the system, all in sterile fashion. Further, the thermocouple wires can provide a nidus for clotting or allow electrical leakage currents to enter the blood. Possibly, the wires might allow pieces of wire to be carried away in the blood.
(d) Radiowave Blood Warmers
The Taurus model 300 radiowave blood warmer is manufactured in England by the Plessey Group. Blood is warmed by radiowave energy. A mixing mechanism consisting of a blood unit between two circular condenser plates which oscillate back and forth through an angle of 120 degrees at a rate of 50 rpm ensures uniform heat distribution. Blood temperature is monitored from the surface of the bag by a probe mounted in the center of one of the condenser plates. Warming is discontinued when the temperature reaches a preset value or if the temperature sensor or the mixing mechanism or the cooling fan is damaged. The device will not operate if there is no blood in the chamber or if the door is open.
The radiowave blood warmer is a batch-type warmer and consequently suffers from the disadvantages of batch warming. Some researchers have reported alterations resembling those of aging of blood which were more obvious after warming with radiowaves than with microwaves.